Remove useless assertion.

Here, snapshot->xcnt is an unsigned type, so it will always be
non-negative.

src/backend/utils/time/snapmgr.c

@@ -1480,7 +1480,6 @@ SerializeSnapshot(Snapshot snapshot, char *start_address)
 {
 	SerializedSnapshotData *serialized_snapshot;
 
-	Assert(snapshot->xcnt >= 0);
 	Assert(snapshot->subxcnt >= 0);
 
 	serialized_snapshot = (SerializedSnapshotData *) start_address;